Deformable non-round membrane assemblies

ABSTRACT

A deformable membrane assembly comprising a fluid-filled envelope, one wall of which is formed by an elastic membrane that is held around its edge by a bendable supporting ring, a support for the envelope and at least three ring-engaging members that are arranged to apply a force to the ring at spaced control points for adjusting the shape of the membrane, with the ring bending to control the shape of the membrane to a predefined form. A control point is provided at or proximate each point on the ring where the profile of the ring that is needed to produce the predefined form of the membrane exhibits a turning point in the direction of the force applied at the control point between two adjacent points where the profile of the ring exhibits an inflection point or a turning point in the opposite direction.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a national stage application, filed under 35 U.S.C.§371, of International Application No. PCT/GB2012/051426, filed on Jun.20, 2012, which claims priority to, and benefit of Great BritainApplication No. 1205394.8, filed on Mar. 27, 2012, each of which isherein incorporated by reference in its entirety.

The present invention provides improvements in or relating to deformablenon-round membrane assemblies in which the shape of a membrane iscontrollably adjustable by altering the fluid pressure across themembrane. The invention has particular reference to assemblies in whichthe membrane is selectively deformable spherically or according toanother Zernike polynomial. In some embodiments the assembly may be avariable optical power fluid-filled lens in which the membrane istransparent and forms one optical surface of the lens whose curvaturecan be adjusted over substantially the entire lens with minimal opticaldistortion that would otherwise be caused by the non-round character ofthe lens. In other embodiments, the membrane may be mirrored and/oropaque. Other applications of the membrane assembly include acoustictransducers and the like.

Variable focus fluid-filled lenses are known in the art. Such lensesgenerally comprise a fluid-filled transparent envelope, the oppositeoptical surfaces of the lens being formed by two spaced opposing wallsof the envelope, at least one of which walls comprises a flexibletransparent membrane. For example, U.S. Pat. No. 1,269,422 discloses alens with spaced opposed walls of arcuate formation that are mergedtogether at their circumferential edges and which may be adjustedtowards or away from each other, and a liquid body between the walls.The pressure of the fluid within the envelope is adjustable to changethe degree of curvature of the membrane, thereby adjusting the power ofthe lens. In some examples, the volume of the envelope may be adjusted,as in U.S. Pat. No. 1,269,422 or WO 99/061940 A1. Alternatively theamount of fluid within the envelope may be adjusted, as in U.S. Pat. No.2,576,581, U.S. Pat. No. 3,161,718 and U.S. Pat. No. 3,614,215. Ineither case, an increase in the fluid pressure within the envelopecauses deformation of the flexible membrane.

Whilst various applications of adjustable lenses are possible—forexample in cameras and other optical equipment—, one use is in eyewear.An adjustable lens is particularly useful for correction of presbyopia—acondition in which the eye exhibits a progressively diminished abilityto focus on close objects with age. An adjustable lens is advantageousbecause the wearer can obtain correct vision through a range ofdistances from long-distance to near vision. This is more ergonomic thanbifocal lenses in which near-vision correction is provided in a bottomregion of the lens, thereby only allowing the user to see close objectsin focus when looking downwardly.

A disadvantage of the fluid-filled lenses disclosed by the documentsmentioned above is that they need to be circular, or at leastsubstantially circular, with a rigid boundary, in order to maintain themembrane spherical; otherwise unwanted optical distortion occurs.However, circular is not a preferred shape for certain applications,including eyewear, because it is not considered to be aestheticallyappealing for those applications. Round lenses may also be unsuitable orunpractical for certain applications, such as in optical instruments.

It is desirable therefore to provide an adjustable non-round lens, inwhich the lens is not distorted as the optical power of the lens isincreased.

U.S. Pat. No. 5,371,629 discloses a non-circular variable focal lengthlens which includes a rigid lens to provide the wearer's distancecorrection, and a liquid-filled lens bounded by a distensible stretchedelastomeric membrane to provide a variable near addition. The liquid,which has a fixed volume, is stored in the field of view between theelastic membrane and the rigid lens. Variation of the optical power ofthe liquid-filled lens is achieved by displacement of the membranesupport to which the outer periphery of the stretched elastomericmembrane is attached. U.S. Pat. No. 5,371,629 claims that the shape ofthe distended membrane is substantially spherical, despite thecircumference of the membrane being non-circular, by allowing themembrane support to bend in a predetermined controlled manner as it ismoved. Specifically, the thickness of the membrane support varies aroundthe circumference of the membrane support. U.S. Pat. No. 5,371,629asserts that by properly proportioning the moment of inertia of thesection of the membrane support around its circumference, the shape ofthe membrane support, when deflected, can be made such as to result in asubstantially spherical membrane, despite the fact that the freemembrane shape is not circular. The configuration of the membranesupport required to result in the desired deformation for any particularlens can be calculated using the method of finite element analysis or inother ways. However, the liquid-filled lens of U.S. Pat. No. 5,371,629is unpractical for various reasons and was never commercialised. Inparticular, despite its teachings, U.S. Pat. No. 5,371,629 fails todisclose a liquid-filled lens that avoids unwanted distortion when themembrane is distended, and the degree of distortion encountered in theliquid-filled lens of U.S. Pat. No. 5,371,629 renders the lens unusable.

WO 95/27912 A1 proposes a workaround that comprehends the use of anon-round membrane supporting ring having a circular central opening,but this does not provide a true non-round lens and is a cumbersomearrangement that is also sub-optimal from an aesthetic point of view.

Similarly, it is desirable to be able to adjust controllably the shapeof a membrane for other non-optical applications. For example, a surfaceof controllably variable sphericity or some other Zernike polynomialwould be useful in the field of acoustics for the creation of non-roundtransducers, such as loudspeakers. Many products would benefit fromnon-round drivers owing to space constraints and the typical geometry ofthe product, e.g. televisions, mobile phones. Maintaining the sphericityof a membrane of variable curvature would be beneficial in theproduction of drivers, since spherical deformation would ensure theemitted waves behave as though they originated from a point source,thereby avoiding interference patterns in the emitted pressure waves.However, the unmodified deformed shape of a non-round membrane that isheld at its edges is not spherical. Hence providing a selectivelyadjustable non-round surface would be desirable for improving theperformance of non-round drivers for acoustic use.

In one aspect of the present invention therefore there is provided adeformable membrane assembly as claimed in claim 1 below.

The present inventors have realised that in a deformable membraneassembly such, for example, as a fluid-filled lens in which the flexibleenvelope contains a fixed volume of fluid and the membrane is distendedto adopt a predefined form by adjusting the volume of the envelope, toalter the pressure of the fluid therein, the control points where theforce is applied to the membrane supporting ring for adjusting theenvelope volume must be carefully positioned. By controlling carefullythe control points at which the force is applied to the membranesupporting ring and allowing the membrane supporting ring to bend freelybetween the control points, semi-active control over the shape of themembrane is achieved. The bending stiffness of the supporting ringvaries around the ring so that when actuated the ring conforms to thedesired profile to produce a membrane shape of the predefined form.Suitably the bending stiffness may be varied round the ring by varyingthe second moment of area of the ring.

The means for causing relative movement between the supporting ring andthe support for the envelope for adjusting the volume of the envelopemay comprise means for moving the supporting ring or support. Saidmoving means may be configured for compressing the envelope to reduceits volume, thereby to increase the pressure of the fluid within theenvelope and to cause the membrane to distend outwards relative to theenvelope in a convex manner. Thus in some embodiments the moving meansmay be configured for compressing the envelope in a first directionagainst the support to increase the pressure of the fluid therein tocause the membrane to distend outwardly in a second opposite direction.

In another aspect of the present invention therefore there is provided adeformable membrane assembly as claimed in claim 6 below.

Alternatively the means for moving the supporting ring or support foradjusting the volume of the envelope may be configured for expanding theenvelope to increase its volume, thereby to reduce the pressure of thefluid within the envelope and to cause the membrane to distend inwardsin a concave manner.

The means for moving the supporting ring or support for adjusting theenvelope volume may suitably comprise a selectively operable devicecomprising one or more components arranged to act between the membranesupporting ring and the support to move the supporting ring and/or thesupport, the one relative to the other, to adjust the volume of theenvelope.

Suitably the flexible envelope may comprise the one wall defined by themembrane and another opposing rear wall that is joined to the edge ofthe membrane in such a manner as to close and seal the envelope. In someembodiments, the opposing walls may be joined directly to one another.Alternatively the envelope may comprise a peripheral side wallintermediate the two opposing walls. The side wall may be flexible toallow the opposing walls to be moved towards or away from each other foradjusting the volume of the envelope. The rear wall may be rigid orsubstantially rigid or may be supported stably at least round aperipheral edge.

The means for moving the supporting ring or support may be configured toact between the membrane supporting ring and the rear wall. In someembodiments, the rear wall may form part of the support for theenvelope, in that the rear wall may afford a stable part for theadjusting means to react against.

The invention is especially applicable to non-round membranes in whichthe edge of the membrane is planar in the un-actuated state and deviatesfrom the planar when the assembly is actuated. However the invention isequally applicable to round membranes where, by dint of the shape of thepredefined form, the edge of the membrane similarly deviates from theplanar when the assembly is actuated. In particular the invention isalso concerned with round membranes where the predefined form isnon-spherical.

To produce the predefined membrane form when actuated, the supportingring must adopt an actuated profile in which one or more regions of thering are displaced in one direction away from a planar datum defined bythe ring in the un-actuated state and/or one or more regions must bedisplaced from the planar datum in another opposite direction. Toachieve the desired actuated profile a force is applied to thesupporting ring at each control point. The inventors have realised thatthere should be at least one control point within each sector of thesupporting ring, whereby a “sector” is meant a portion of the ring lyingbetween two adjacent inflection points or minimal points in the profile,said minimal points being local minimums of displacement of the ring inthe direction of the force applied at the control point, e.g. in thefirst direction inwards relative to the envelope, when the membrane isdeformed. Since a minimal point is defined as being a local rather thana global minimum in the direction of the applied force at the adjacentcontrol points (and thus a local rather than a global maximum in thedirection opposite to the direction of the applied force, e.g. in thesecond direction outwards relative to the envelope) it will beunderstood that at these points the ring may actually be displaced ineither direction, or not displaced at all, from the planar datum. Ingeneral the ring may at all points move in either direction from, orremain stationary at, the planar datum, depending on the perimetershape, surface profile and actuation required. In some embodiments whereforces having opposite directions are applied at adjacent control pointsto achieve a desired ring profile when the membrane is deformed, acontrol point may be positioned between two inflection points in theprofile of the supporting ring. However the forces applied at thecontrol points will usually all be in the same direction, such that asector of the ring is defined between adjacent local minima as describedabove.

In some embodiments, the ring may be non-round and the predefined formmay have a centre. In such embodiments, the minimal points of minimaldisplacement may also be minimal points in the sense that the distancebetween the supporting ring and the centre of the predefined form of themembrane when distended is a local minimum. It will be understood thatthe position of the centre will depend on the shape of the predefinedform. In some embodiments the centre may be at or close to the geometriccentre of the membrane. Alternatively the centre of the predefined formmay be at a different location from the geometric centre of themembrane. Typically, when deformed, the membrane will have a vertex(i.e. a point of global maximum displacement) and the centre may belocated at the vertex. This is particularly the case in opticalapplications where the membrane forms an optical surface of the lens.Generally the centre of the defined form will be positioned somewherewithin the body of the membrane away from the supporting ring.

In practice, according to the shape of the membrane, some regions of thering may be supported to reduce the flexibility of the ring in thoseregions. Accordingly, the inventors have realised that there should beat least one control point within each sector of the ring betweenunsupported minimal points. It will be appreciated that the number ofsuch minimal points will depend upon the shape of the ring. In someembodiments, the number of minimal points may be determined by thenumber of corners of the ring. For instance, a quadrilateral ring withfour corners has four minimal points generally equidistant between thecorners where the centre of the predefined form of the membrane is at ortowards the geometric centre of the quadrilateral. In practice, thecentre may be positioned asymmetrically between opposite sides, and suchan arrangement may be particularly suitable for a rectangular opticallens. In some embodiments, in a quadrilateral shaped ring, the centre ofthe defined form may be positioned generally symmetrically between onepair of opposite sides and asymmetrically between the other pair ofopposite sides.

In a generally rectangular ring with two long sides and two short sidesthere will normally be four such minimal points where displacement ofthe ring from the planar datum in the direction opposite to thedirection of the force applied to the supporting ring at the adjacentcontrol points is a local maximum, one on each of the sides between twoadjacent corners, but in some embodiments, especially where the shortsides are substantially shorter than the long sides, the short sides ofthe ring may be reinforced to reduce their flexibility, so that inpractice the ring along each short side does not bend substantially asthe membrane is distended, in which case there are only two unsupportedminimal points along the two long sides. In such a rectangular ring, foroptical applications, the centre of the defined form may be positionedfurther from one short side than from the other.

The inventors have also realised that there should be at least threecontrol points, regardless of the number of minimal points and sectorsin order to define the plane of the membrane.

Further, the inventors have realised that within each sector, a controlpoint should be positioned at or close to a maximal point where thedisplacement of the ring in the actuated state away from the planardatum in the direction of the force applied at the control point in thatsector is a local maximum, e.g. in the first direction inwards relativeto the envelope to achieve compression of the envelope. It will beunderstood that where the rest of the ring within a given sector isdisplaced in the opposite direction when actuated, e.g. in the seconddirection outwardly relative to the envelope, the maximal point withinthat region may be a point at which the ring is stationary, i.e. issubject to no or substantially no displacement away from the planardatum. Further, a maximal point may be a point at which the ring isactually displaced in the opposite direction from the planar datum,e.g., outwardly relative to the envelope, less far than the rest of thering within the same sector. In other words a point of locally maximaldisplacement in the direction of the force applied at the control pointis equivalent to a point of local minimum displacement from the planardatum in the opposite direction.

In embodiments where the ring is non-round and the predefined form has acentre, a maximal point may be a point on the ring between adjacentinflection or minimal points where the distance between the ring and thecentre of the predefined form of the membrane when distended is amaximum. If this were not the case then within a sector there would be aportion of the ring that was further away from the centre than thecontrol point(s) within the sector and which would therefore beuncontrolled, leading potentially to unwanted distortion and shape ofthe membrane when distended.

In some embodiments, one or more of said control points may be actuationpoints, where the ring-engaging members are configured actively todisplace the supporting ring relative to the support. Said supportingring may be formed with a protruding tab at the or at least one of theactuation points for engaging the ring with the ring engaging element.

The membrane may be continuously adjustable between an un-actuated stateand fully distended state. The supporting ring may be planar whenun-actuated.

At each position between the un-actuated and fully distended states thesupporting ring may be displaced at the or each actuation point by thedistance required to achieve the profile required to produce thepredefined form of the membrane. This is important so that at eachposition between the un-actuated and fully distended states, the ring ispositioned at the or each actuation point at its desired location withinthe overall desired profile of the ring. It will be understood that ifthe actuation point were to be held in a different position by the ringengaging member at that point then local distortion in the desiredprofile of the ring would occur at that point leading potentially tounwanted distortion in the shape of the membrane.

In some embodiments, one or more of said control points may be hingepoints, where the ring-engaging members are configured to hold thesupporting ring stationary relative to the support. The supporting ringis required to remain stationary at the or each hinge point to achievethe required actuated ring profile to produce the predefined form of themembrane at each position between the un-actuated and fully distendedstates. Thus, in the same way as the actuation points, the ring must beheld at each hinge point by the ring-engaging member at that point in aposition that corresponds to the desired overall profile of the ring ateach state of the ring between the un-actuated and fully distendedstates. Since the ring is not displaced at each hinge point, it followsthat the position of the ring at each hinge point must be the same foreach state of the ring between the un-actuated and fully distendedstates. Where the predefined form has a centre, there may be a pluralityof hinge points that are substantially equidistant from the centre ofthe predefined form.

In some embodiments two adjacent hinge points may define a tilting axis,in which case there is suitably at least one actuation point where thering engaging member is configured actively to displace the supportingring relative to the support for tilting the ring relative to thesupport about said tilting axis in the first direction for compressingor the second direction for expanding the envelope.

For some applications, the supporting ring may be generally rectangular,having two short sides and two long sides. In such cases, at least oneactuation point may be located on one of the short sides, and twoadjacent hinge points may be located on or proximate to the other shortside. The predefined form may have a centre which may be locatedoff-centre with respect to the membrane, being closer to the other shortside than it is to the one short side. The one short side may generallyfollow the arc of a circle that is centred on the centre of the definedform. The at least one actuation point may be located substantiallycentrally on said one short side.

The supporting ring should be free to bend passively relative to thesupport intermediate the control points. However, in some embodiments itmay be desirable to control the bending of the ring by means ofstiffening elements for stiffening one or more regions of the supportingring.

Advantageously the supporting ring may comprise two or more ringelements, and the membrane may be sandwiched between two adjacent ringelements.

According to another aspect of the present invention therefore there isprovided a deformable membrane assembly as claimed in claim 18 below.

Suitably, the membrane may be pre-tensioned on the membrane supportingring. The inventors have realised that by sandwiching the membranebetween two adjacent ring elements, the torsional forces applied by themembrane to the ring can be balanced out resulting in no orsubstantially no net torsional force. It will be appreciated that it isdesirable to avoid torsional forces on the ring which might lead tounwanted distortion in the shape of the ring and thus in the shape ofthe membrane when distended. Thus, in some embodiments, the membranesupporting ring may consist of two ring elements. In some embodimentsmore than two ring elements may be provided. However, the arrangementshould be such that when the membrane is pre-tensioned between the twoadjacent ring elements, the torsional forces on the ring elements aboveand below the membrane cancel each other out or substantially canceleach other out.

The means for adjusting the pressure within the envelope may comprise aselectively operable device comprising one or more components arrangedto adjust the fluid pressure in the envelope. In some embodiments, themeans for adjusting the pressure of a fixed volume of fluid within themembrane may comprise means for compressing or expanding the envelope asmentioned above. Suitably, a fixed support may be provided, and meansmay be provided for compressing or expanding the envelope against thesupport to increase or decrease the pressure of the fluid therein.

Suitably the supporting ring may have a substantially uniform depth anda variable width to control the second moment of area round the ring andthus the bending stiffness of the ring. Typically the supporting ringmay be narrowest where it is required to bend the most to achieve thepredefined form when the membrane is distended.

In some embodiments the predefined membrane shape may be spherical oranother form defined by one or more Zernike polynomials. These have thegeneral formula Z_(n) ^(±m). Various shapes, as defined by Zernikefunctions or combinations of more than one such function, are possibleusing the lens assembly of the present invention. A priority forophthalmic applications, for instance, is to be able to achieve visioncorrection with a linear superposition of Z₂ ^(±2) (astigmatism) and Z₂⁰ (sphere for distance correction). Opticians typically prescribe lensesbased on these formulae. Higher order surfaces with additionalcomponents Z_(j) ^(±j) are also possible if additional control pointsare provided on the edge of the membrane, where j scales in similarmagnitude to the number of control points. Higher order surfaces withcomponents Z_(j) ^(±k) (k≦j) may also be possible where the shape of themembrane edge permits.

Further, various linear superpositions of scaled Zernike polynomials ofthe form Z_(n) ^(±m) are possible:Z ₂ ^(±2) , Z ₂ ⁰ , Z _(j) ^(±j) , Z _(j) ^(±k) (k≦j)

In general, except at their periphery, surfaces achievable by deforminga membrane with pressure may have one or more local maxima or one ormore local minima, but not both, in addition to saddle points. Theshapes that are achievable are necessarily limited by the shape of theperiphery, which is stable in use.

Suitably, the required bending stiffness round the ring may bedetermined by finite element analysis (FEA). In particular, FEA may beused to calculate the required variation in bending stiffness round thering for the ring to adopt the desired profile when actuated in order toproduce a membrane shape of the predefined form. For quasi-static or lowfrequency optical and other applications, static FEA should be employedadequately. However, where the surface is intended for acousticapplications, dynamic FEA may be appropriate. FEA—whether static ordynamic—involves numerous iterations performed using a computer with theinput of selected parameters to calculate the membrane shape that wouldresult in practice with an increasing force applied at the controlpoints. The element shape may be selected to suit the calculation beingperformed. The selected parameters to be input may include the geometryof the supporting ring, the geometry of the membrane, the modulus of themembrane, the modulus of the ring, including how the modulus of the ringvaries round the ring (which may be defined empirically or by means of asuitable formula), the amount of pre-tension in any of the parts, thetemperature and other environmental factors. The FEA programme maydefine how the pressure applied to the membrane increases as load isapplied to the rings at the control points.

Within each iteration of the FEA the calculated shape of the membrane iscompared with the predefined form, and any deviation between thecalculated shape the predefined form used to adjust the bendingstiffness round the membrane supporting ring for the next iteration.Progressively, the bending stiffness of the supporting ring is adjustedso that the calculated shape of the membrane converges with the desiredpredefined form.

A reinforcing diaphragm may be provided that is fastened to thesupporting ring, which diaphragm has a greater stiffness in the plane ofthe ring than in the direction of bending of the ring.

In yet another aspect of the present invention therefore there isprovided a deformable membrane assembly.

As mentioned above, the membrane is suitably pre-tensioned on themembrane supporting ring. The reinforcing diaphragm serves to stiffenthe ring in the plane of the membrane in the un-actuated state againstthe additional loading that is created by the pre-tensioning within themembrane, while allowing the ring to bend freely in the direction normalto the ring. Alternatively the supporting ring itself may have a greaterbending stiffness in the plane of the membrane in the un-actuated statethan out of the plane of the membrane.

Suitably, the reinforcing diaphragm may be fastened to the supportingring uniformly round the ring so that the tension in the membrane istransmitted uniformly to the diaphragm.

In some embodiments, in the plane of ring, the membrane may be longer inone dimension than it is in another dimension. In such cases, thereinforcing diaphragm may have a lower stiffness in the one dimensionthan it has in the other dimension. Alternatively the geometry of theassembly may itself serve to may be used to compensate for theconsequent differential strain in the membrane.

The means for adjusting the pressure within the envelope may comprise aselectively operable device comprising one or more components arrangedto increase or decrease the fluid pressure in the envelope. Typicallythe means for adjusting the pressure within the fluid-filled envelope,which may contain a fixed volume of fluid, may comprise means forcompressing or expanding the envelope. The fluid-filled compressibleenvelope may comprise an at least partially rigid rear wall that isspaced from the distensible membrane and a flexible side wall betweenthe membrane and the rear wall.

In some embodiments, the membrane, rear wall and fluid are transparentsuch that the membrane and rear wall form an adjustable optical lens.Where provided, the reinforcing diaphragm may also be transparent.

Suitably said rear wall may be shaped to provide a fixed lens.

The assembly may further comprise a protective rigid front cover overthe membrane. The front cover may be transparent. Suitably the frontcover may be shaped to provide a fixed lens.

Thus, in some embodiments, the front cover and/or rear cover may providea fixed optical power for the correction of refractive errors such asmyopia and hyperopia. The adjustable optical lens of the invention maybe used to provide an additive (or subtractive) optical power to thefixed optical power of the front or rear lens for the correction ofpresbyopia. Suitably the front and/or rear lenses may be shaped for thecorrection of astigmatism, and similarly the predefined form of thedistended membrane of the adjustable optical lens of the invention maybe adapted for the correction of astigmatism.

In some embodiments the envelope may be housed within a retaining ring.

In yet another aspect of the present invention there is provided anarticle of eyewear comprising a deformable membrane assembly inaccordance with the invention.

Said article of eyewear may typically comprise a frame with a rimportion; the deformable membrane assembly may be mounted within the rimportion.

Following is a description by way of example only with reference to theaccompanying drawings of embodiments of the present invention.

In the drawings:

FIG. 1 is a perspective view from above of the front of a pair ofeyeglasses comprising a frame that is fitted with two first lensassemblies according to the invention;

FIG. 2 is a perspective view from above and to the left of the left handside of the eyeglasses of FIG. 1 showing how one of the first lensassemblies is fitted to the frame;

FIG. 3 is a front elevation of the first lens assembly in accordancewith the invention in the un-actuated state;

FIG. 4 is a cross-section of the first lens assembly along the lineIV-IV of FIG. 3;

FIG. 5 is a cross-section of the first lens assembly along the line V-Vof FIG. 3;

FIG. 6 is a cross-section of the first lens assembly along the lineVI-VI of FIG. 3;

FIG. 7 is a perspective view from below and to the left of the front ofthe first lens assembly of the invention which is shown cut-away alongthe line VI-VI of FIG. 3;

FIG. 8 is an exploded view of the first lens assembly showing the partsof the assembly;

FIG. 9 is a front elevation of the flexible membrane and membranesupporting rings of the first lens assembly in the un-actuated state,showing how the width of the rings varies round the periphery of themembrane to control the second moment of area of the rings;

FIG. 10 shows the membrane and rings of FIG. 9 in an actuated state andprojected onto a notional sphere of radius R;

FIG. 11 is a cross-section of the first lens assembly which correspondsto FIG. 4 but shows the assembly in an actuated state;

FIG. 12 is a cross-section of the first lens assembly which correspondsto FIG. 5 but shows the assembly in an actuated state;

FIG. 13 shows the displacement of the membrane of the first lensassembly in an actuated state, as calculated by static finite elementanalysis (FEA);

FIG. 14 shows the uniformity of the optical power of the first lensassembly in an actuated state, as calculated by FEA;

FIG. 15 shows the variation of pre-tension in the membrane as calculatedby FEA of a lens assembly in the un-actuated state that is similar tothe first lens assembly but omits the reinforcing diaphragm;

FIG. 16 shows the variation of pre-tension in the membrane as calculatedby FEA of the first lens assembly of the invention in the un-actuatedstate;

FIG. 17 shows the variation in the optical power as calculated by FEA ofa lens assembly in an actuated state that is similar to the first lensassembly but omits the reinforcing diaphragm;

FIG. 18 shows the variation in the optical power as calculated by FEA ofthe first lens assembly of the invention;

FIGS. 19A-C show schematically in cross-section the first lens assemblyof the invention in the un-actuated state (FIG. 19A), an actuated state(FIG. 19B) and a de-actuated state (FIG. 19C);

FIGS. 20A-C show schematically the front elevation of the first lensassembly of the invention in the un-actuated state (FIG. 20A), anactuated state (FIG. 20B) and a de-actuated state (FIG. 20C);

FIGS. 21A-C show schematically in cross-section a second square lensassembly of the invention in an un-actuated state (FIG. 21A), anactuated state (FIG. 21B) and a dc-actuated state (FIG. 21C);

FIGS. 22A-C show schematically the front elevation of the second lensassembly of the invention in the un-actuated state (FIG. 22A), anactuated state (FIG. 22B) and a de-actuated state (FIG. 22C);

FIG. 23 shows how the distance between the optical centre and themembrane supporting rings varies in the first lens assembly;

FIG. 24 shows how the distance between the optical centre and themembrane supporting rings varies in the second lens assembly of FIGS.21A-C and FIGS. 22A-C.

FIG. 25 shows schematically in cross-section a flexible membrane andsingle supporting ring in accordance with the invention; and

FIG. 26 shows schematically in cross-section the flexible membrane andsupporting rings of the first lens assembly in accordance with theinvention.

With reference to FIG. 1, a pair of eyeglasses 90 (UK: spectacles)comprise a frame 92 having two rim portions 93 and two temple arms 94.The rim portions 93 are joined together by a bridge 95, and each isshaped and dimensioned to carry a respective first lens assembly 1 inaccordance with the present invention. One of the first lens assemblies1 is used for the right side of the eyeglasses, and the other is usedfor the left side. As can be seen from FIG. 1 the right-hand andleft-hand lens assemblies 1 are mirror images of one another, but theirconstruction is otherwise identical, and therefore only the left-handside one is described in detail below, but it will be appreciated thatthe construction and operation of the right-hand side one is the same.

As best seen in FIG. 3, the first lens assembly 1 has a generallyrectangular shape with two opposing long sides 3, 5 and two short sides7, 9 and is designed to fit with the frame 92, but it will beappreciated that the shape of the first lens assembly shown is only oneexample of a suitable shape, and a lens assembly in accordance with theinvention may be given any shape that is desired. The invention isespecially suited for non-round shapes, such as the one shown in FIGS. 1and 3, but the teachings of the invention may also be applied to roundlenses. In round lenses, the invention may be used, by way of example,for the correction of aberrations in an optical system requiring morethan spherical wave-front correction.

As well as eyeglasses, the lens assembly of the present invention isequally well applicable to other lens applications, such as goggles,helmets and scientific and optical instruments of various sorts. In thelens assembly 1 the optical parts as described below are transparent,but the invention also comprehends other kinds of deformable membraneassemblies which are constructed and operate in a similar manner toprovide a controllable adjustable surface, and thus such membraneassemblies in accordance with the invention may also find application innon-optical fields, such as acoustics where a surface with a selectivelyand controllably adjustable shape may be required.

The first lens assembly 1 is especially suitable for use in thecorrection of presbyopia. In use, the first lens assembly 1 can beadjusted for bringing into focus objects at a range of distances fromlong distance to close distance. In this embodiment there is nocorrection provided for long distance, but nevertheless, the first lensassembly 1 allows a user to re-focus smoothly from a far-away object toa near, reading-distance object.

The first lens assembly 1 comprises a pair of membrane supporting rings2, 10 of uniform thickness but variable width. The design of these ringsis explained in more detail below. A retaining ring 6 holds the parts ofthe first lens assembly 1 together.

In FIG. 8, the component parts of the first lens assembly 1 can be seenin exploded view. The front of the first lens assembly 1 is shown at thetop right of the figure, and the rear of the assembly (which in usewould be closest to the wearer's eye) is at the bottom, although it willbe appreciated that all the other parts fit into the retaining ring 6,which forms an enclosing housing for said other parts.

At the front of the first lens assembly 1 is a transparent front coverplate 4, made of glass or a suitable polymeric material. In the firstlens assembly the front cover plate is about 1.5 mm thick, but this maybe varied as mentioned below. Further, in some embodiments, as describedbelow, the front cover plate 4 may comprise a lens of fixed focalpower(s), for example a single vision (single power), multi-focal (twoor more powers), progressive (graded power) or even an adjustableelement. As shown in FIG. 4 for example, in the present embodiment, thefront cover plate 4 is piano-convex.

Behind the front cover plate 4 are disposed two stiffening ribs 3 a, 3b, which provide extra stiffness at the short sides 7, 9 of the firstlens assembly 1, as described in more detail below. Next is a front oneof the pair of resiliently bendable supporting rings 2. The rings may bemade of stainless steel and, in the first assembly, are about 0.3 mmthick, but other suitable materials may be used and the thicknessadjusted accordingly to provide the desired stiffness as discussedbelow. Next is a transparent non-porous, elastic membrane 8. In thefirst assembly the membrane 8 is made of Mylar® and is about 50 micronsthick, but other materials with a suitable modulus of elasticity may beused instead. Behind the membrane 8 is disposed a rear one of the pairof bendable supporting rings 10 of substantially the same geometry asthe front supporting ring 2. The flexible membrane 8 is pre-tensioned asdescribed below and attached to and sandwiched between the front andrear supporting rings 2, 10, such that it is stably supported around itsedge, as shown in FIGS. 3-7 in which the first lens assembly 1 is shownin its assembled condition. The membrane 8 forms a fluid-tight seal withat least the rear supporting ring 10.

The rear surface of the second supporting ring 10 is sealed to atransparent reinforcing diaphragm 24. In the first embodiment thereinforcing diaphragm 24 may comprise a sheet of polycarbonate, butalternative materials that are suitable to provide the requiredproperties as described below may be used instead. Behind said diaphragmis a dish-shaped part 12 having a flexible side wall 18, a rear wall 19and a forward sealing flange 20. In the first assembly the dish-shapedpart 12 is made of transparent DuPont® boPET and is about 6 micronsthick, but other suitable materials for the dish-shaped part may be usedand the thickness adjusted accordingly. The forward sealing flange 20 ofthe dish-shaped part 12 is sealingly adhered to the rear surface of thediaphragm 24 with a suitable adhesive such, for example, as Loctite3555.

A layer of a suitable transparent pressure-sensitive adhesive (PSA)such, for example, as 3M® 8211 (not shown) adheres the rear wall 19 ofthe dish-shaped part 12 to a front face 17 of a transparent rear coverplate 16 having a rear face 14. In the first lens assembly 1 describedherein the layer of PSA is about 25 microns thick, but this may bevaried as required. The rear cover plate 16 may be made of glass orpolymer and in the first assembly 1 is about 1.5 mm thick, but againthis may be varied as desired. The rear cover plate 16 sits as therearmost layer within the retaining ring 6. As with the front coverplate 4, in some embodiments, the rear cover plate 16 may form a lens ofa fixed focal power. In the present embodiment, as seen in FIG. 4 forexample the rear cover plate 16 is a meniscus lens.

The retaining ring 6 comprises a forwardly extending side wall 13 havingan inner surface 23, which side wall 13 terminates in a front edge 15.The front cover plate 4 sits on and is bonded to the front edge 15 ofthe retaining ring 6 so that the lens assembly constitutes a closedunit. As best seen in FIGS. 4, 5, 11 and 12, the cover plate 4 is spacedforwardly of the front membrane supporting ring 2 to provide a spacewithin which the membrane 8 may distend forwardly in use as describedbelow without impinging on the front cover plate.

The dish-shaped part 12, membrane 8, second supporting ring 10 anddiaphragm 24 thus define a sealed interior cavity 22 for holding atransparent fluid. For optical applications, such as the first lensassembly 1 described here, the membrane 8 and the rear face 14 of therear cover plate 16 form the opposite optical surfaces of an adjustablelens. As described above the rear cover plate 16 is a meniscus lens. Inan un-actuated state, the membrane is planar, so the lens has the fixedoptical power afforded by the rear cover plate 16, with zero additionfrom the membrane 8. However, when actuated as described below, themembrane 8 is inflated to protrude forwardly in a convex configurationand thus adds positive optical power to the fixed meniscus lens. In someembodiments, the membrane may distend inwardly in a concaveconfiguration such that in combination with the rear face 14 of the rearcover plate 16, the lens 1 is biconcave. The greater the curvature ofthe membrane 8, the greater the additional optical power afforded by themembrane 8. For non-optical applications the fluid, along with the otherparts of the assembly, do not need to be transparent.

The side wall 18 of the dish-shaped part 12 provides a flexible sealbetween the rear wall 19 and the diaphragm 24, thus forming the sides ofthe cavity 22. This flexible seal is provided so that there can berelative movement between the supporting rings 2, 10 and the rear coverplate 16 when the first lens assembly 1 is actuated to adjust the powerof the lens. The deformable membrane 8 is adhered to the first 2 andsecond 10 supporting rings, for example by Loctite® 3555.

The cavity 22 is filled during manufacture with a transparent oil 11(see FIG. 7), such for example as Dow Corning DC705, which is chosen tohave an index of refraction as close as possible to that of the rearcover plate 16. The oil 11 is also chosen so as to not be harmful to awearer's eye in the event of a leakage.

As shown in FIGS. 6 and 7, the first lens assembly 1 may be received andseated snugly in a rear rim part 93 b which is shaped and dimensioned tomate with a front rim part 93 a as shown in FIG. 2 to form one rimportion 93 of the frame 92 of the eyeglasses 90. The front and rear rimparts 93 a, 93 b may be fixed together by any suitable means availableto the person skilled in the art. For instance, the front and rear rimparts may be formed with matching screw holes 97 that are adapted toreceive small fixing screws for holding the two rim parts securelytogether and to retain the lens assembly 1 therebetween. In someembodiments, the rear rim portion 93 b may be formed integrally with theretaining ring 6.

In some embodiments the reinforcing diaphragm 24 may be omitted, inwhich case the sealing flange 20 of the dish-shaped part 12 would beattached directly to the rear surface of the rear supporting ring 10.

It will be appreciated that the present invention is not limited to theparticular materials and dimensions given above, which are given only byway of example. Different types of materials may suitably be used forthe dish-shaped part 12 that are optically clear, have low overallstiffness compared with the supporting rings 2, 10 and are joinable tothe diaphragm 24 or rear supporting ring 10.

Various different materials may suitably be used for the supportingrings 2, 10 provided they fulfil the criteria of: having sufficientlyhigh modulus to be able to be made thin relative to the overall depth ofthe first lens assembly 1 (i.e. of the order of 0.3 mm thickness); beingjoinable to the adjacent components; having low creep (to continue toperform over multiple uses); and being elastically deformable. Otherpossibilities are titanium, glass and sapphire. By “joinable” is meantby joinable by adhesive, crimping, laser welding or ultrasonic weldingor any other means that would be apparent and available to those skilledin the art.

Different adhesives may suitably be chosen that are able to join theparts of the assembly durably, are creep resistant, are of a suitableviscosity to be applied when constructing the lens assembly and remaininert in the presence of the fluid in the lens. Particular adhesives maybe chosen in dependence on materials selected for the various parts.

There are various other suitable materials that permit sufficientflexing of the membrane 8, and various colourless oils may be used,particularly in the family of high refractive index siloxane oils forwhich there are a number of manufacturers. The materials chosen for thevarious components need to be such that they provide stability aroundthe hinge and actuation points (described below with reference to FIGS.9 and 10).

The first lens assembly 1 provides an adjustable lens having a focalpower that can be adjusted by controlling pressure of the fluid 11within the cavity 22 and the shape of the bendable supporting rings 2,10, thereby controlling deformation of the elastic membrane 8 into adesired profile. As mentioned above the membrane 8 forms one of theoptical surfaces of the lens, the other one being the rear face 14 ofthe rear cover plate 16. Deformation of the membrane 8 increases thecurvature of the optical surface provided by the membrane and changesthe optical thickness of the lens between the surfaces, therebyincreasing the additional optical power afforded by the membrane 8.Details of this operation are given below.

As best seen in FIG. 9 the width of the supporting rings 2, 10 in thex-y plane normal to the front-rear z-axis of the lens assembly 1 variesin a predetermined manner round the periphery of the assembly 1. This isto provide for the desired deformation of the supporting rings 2, 10which in turn controls deformation of the flexible membrane 8 and hencethe power of the lens, as explained in more detail below.

It can be seen from FIG. 8 that each of the supporting ribs 3 a, 3 b,the supporting rings 2, 10 and the reinforcing diaphragm 24 has aprotruding tab 26 of similar shape and size which protrudes outwardly ofthe first lens assembly 1 from one of the short sides 7 of the assembly1. When assembled, the tabs 26 on these parts are aligned with eachother, and each is formed with one or more closely adjacent holes 28 a,28 b that align with the corresponding holes in the other parts. Theseholes 28 a, 28 b define an actuation point {circle around (A)} forattachment of an actuation device to the lens assembly 1 to cause it tobe compressed in use. Compression of the lens 1 is described in moredetail below. The actuation device may be housed in the adjacent templearm 94 of the frame 92. In some embodiments the lens assembly may beexpanded in a similar manner to reduce the pressure of fluid 11 withinthe cavity 22.

Adjacent the protruding tab 26 at the one short side 7 of the assembly,the inner edge of each of the supporting rings 2, 10 deviates outwardlyas best shown in FIG. 9 to form a generally semi-circular recess 30. Theside wall 18 of the dish-shaped part 12 has a similar, correspondingrecess 30 which aligns with the recesses 30 of the supporting rings 2,10 when the lens is assembled. The membrane 8 includes a correspondingsemi-circular protruding portion 31 that aligns with the recesses 30 toensure the closure of the seal afforded by the membrane. The reinforcingdiaphragm 24 is cut-out at 32 a, which also aligns with the tabs 26.This allows filling of the reservoir 22 after all the parts have beenassembled by protruding beyond the extent of the front and rear coverplates 4, 16. Alternatively as shown in FIG. 10 a separate hole 31′though the supporting rings 2, 10 may be provided instead of saidsemi-circular recess 30.

The reinforcing diaphragm 24 affords significant improvements over priorfluid-filled lenses by dint of its function to stiffen the supportingrings 2, 10 in the plane defined by the rings in the un-actuated state.It is desirable to pre-tension the membrane 8 when assembling the parts,otherwise undesired wrinkles or sag may appear in the membrane owing totemperature and gravitational or inertial effects on the fluid pressureand the like. One way to minimise the risk of such wrinkles or sag wouldbe to support the flexible membrane 8 on an inflexible supporting ring,but this would be incompatible with the need for the supporting rings 2,10 to bend in use. The reinforcing diaphragm 24, which strengthens thesupporting rings 2, 10 in the plane of the membrane 8 to resist bending,but does not significantly add to the stiffness of the rings transversethe membrane (z axis), provides a solution to this problem.

In the first lens assembly 1 described herein, in which the distancebetween the long sides 3, 5 is less than the distance between the shortsides 7, 9—making the first assembly generally rectangular. The lens isthus wider in the E-W direction between the short sides 7, 9 as shown inFIG. 9 than it is in the N-S direction between the long sides 3, 5. Thesupporting rings 2, 10 are configured to bend more along the long sides.It will be appreciated that, when actuated, the membrane 8 is stretchedmore in the E-W direction than it is in the N-S direction, Since thediaphragm 24 can only bend and not distend, it can only bend in onedirection, so it bends along the E-W axis of the lens. Bending a beambrings the two ends of it slightly closer together, and this compensatesfor the differential strain in the membrane 24.

In some embodiments, the diaphragm 24 may be made stiffer in the E-Wdirection than in the N-S direction, and this directional stiffness ofthe diaphragm 24 may be used to compensate for the above-mentioneddifferential strain in the membrane 8.

In the first lens assembly 1, the reinforcing diaphragm 24 is made froma transparent material that is index-matched with the membrane 8 and thefluid 11 within the cavity 22. It comprises a flat sheet that is placedwithin the fluid of the lens between the sealing flange 20 of thedish-shaped part 12 and the rear supporting ring 10, so that it liesbehind the flexible membrane 8 in the assembled lens 1, as best seen inFIGS. 4 and 5. The diaphragm 24 is shaped similarly to the other partsof the lens assembly 1, and in the first assembly is 0.55 mm thick,although this thickness may be varied as desired. Since the diaphragm 24is attached to the dish-shaped part 12 and the rear supporting ring 10round its edge, the stiffness of the supporting rings 2, 10 must beadjusted accordingly such that they are still able to bend as requiredin the z direction transverse the plane of the membrane 8.

The reinforcing diaphragm 24, in accordance with the invention, has beenfound to work better than, for example, localised support of thesupporting rings 2, 10. In one embodiment, the supporting ring size andstiffness may be reduced by approximately 25% as compared with the sizeand stiffness of similar supporting rings 2, 10 that are stiff enough bythemselves to prevent wrinkles without an associated diaphragm 24. Thenecessary ability of the supporting rings 2, 10 to flex to control thedeformation of the flexible membrane 8 is not impaired. A suitablematerial for the support disc 24 is polycarbonate, but other materialsmay suitably be used. The reinforcing diaphragm 24 of the invention isequally suitable for use in round lenses as it is for non-round lenses,but in such other embodiments the diaphragm does not necessarily need tohave differential stiffness on different axes.

The design of the reinforcing diaphragm 24 is such that its main effectis to increase the stiffness of the supporting rings 2, 10 in thein-plane direction normal to the front-rear axis of the assembly (x-yplane in FIG. 10), but has only a small effect on the bending stiffnessin the z direction (i.e. normal to the rear wall 19). This z-directioneffect is accounted for in the design of the supporting rings 2, 10.Thus the stiffness of the assembly 1 is increased for the purpose ofmaintaining tension in the flexible membrane 8, but the supporting rings2, 10 can still bend in the z direction in use. This may be achieved bychoosing for instance a fibre material which has stiffness in the x-yplane but little stiffness in the z-direction owing to the orientationof the fibres. The diaphragm 24 is formed with a plurality of apertures32 a, 32 b; in the first lens assembly 1 described herein there aretwo—one adjacent the aforementioned tab 26, and the other in a corner ofthe other opposite short edge 9 of the assembly. The materialsurrounding the apertures 32 a, 32 b provides the stiffness, but theapertures 32 a, 32 b allow fluid to pass through and hence have littleor no effect on deformation of the flexible membrane 8. The precisenumber, size and arrangement of the apertures 32 a, 32 b may be variedas desired—for example a plurality of smaller apertures spaced acrossthe diaphragm 24 may be provided. The diaphragm 24 does not deform withthe flexible membrane 8, and the support it provides for the membrane 8is not needed when the lens is in an actuated state with the membranedistended as described below. In the first lens assembly 1 thereinforcing diaphragm 24 comprises a continuous sheet that is formedwith a number of apertures 32 a, 32 b as described above, but in otherembodiments, the diaphragm may comprise a reticulated sheet or a mesh orthe like, as long as it is joined to the supporting rings 2, 10 roundsubstantially their whole extent in order to provide the desiredin-plane stiffness. The diaphragm may be connected to the rings 2, 10substantially continuously or at spaced locations around its peripheryprovided that the load is distributed uniformly without giving rise toany significant local distortion of the rings or membrane 8. Innon-optical applications, there is no need for the diaphragm to betransparent.

As best seen in FIG. 6 the inner surface 23 of retaining ring 6 isformed with two spaced circumferential shelves 34, 36: a rear shelf 34and a forward shelf 36. The rear shelf 34 is disposed proximate the rearof the retaining ring 6; the rear cover plate 16 is supported on saidrear shelf. The forward shelf 36 is disposed intermediate the front edge15 of the retaining ring 6 and serves to support the diaphragm 24 andfront and rear supporting rings 2, 10. The side wall 18 of thedish-shaped part 12 is dimensioned such that its front sealing flange 20is supported on the forward shelf 36 when the lens is assembled.

At said other short side 9 of the first lens assembly 1, the retainingring 6 defines two hinge points {circle around (H)}1, {circle around(H)}2—see FIG. 10. As shown in FIG. 4, the stacked parts 2, 3 b, 8, 10,12, 24 are held in place within the retaining ring 6 by means offormations 39 formed integrally with the retaining ring 6 at the hingepoints {circle around (H)}1, {circle around (H)}2, such that they remainstable when the lens is actuated as described below.

The supporting rib 3 b provides additional stiffness for the supportingrings 2, 10 in the region of the hinge points {circle around (H)}1,{circle around (H)}2 and between them. In the first lens assembly 1, thehinge points {circle around (H)}1, {circle around (H)}2 and the regionof the supporting rings 2, 10 between them are approximately equidistantfrom the optical centre OC of the lens when actuated (see FIG. 10), andso the rings 2, 10 intermediate the hinge points {circle around (H)}1,{circle around (H)}2 are not required to bend much or at all. The othersupporting rib 3 a similarly provides additional stiffness for thesupporting rings 2, 10 at the aforementioned actuation point {circlearound (A)} so that deformation of the membrane 8 is properlycontrolled, as explained in more detail below. In some embodiments thesupporting ribs 3 a, 3 b may be omitted; they are generally useful forregions of the supporting rings 2, 10 that are not required to deformsignificantly during actuation of the assembly.

The shape of the first lens assembly 1 is suitable for the eyeglasses 90in terms of its aesthetic appearance. However, a non-round lens givesrise to the problem of non-uniform, or undesired, deviation from thedesired shape of deformation of the membrane, which would occur in theabsence of a solution to the problem. The means by which the presentinvention addresses and solves this problem is explained below.

FIG. 10 illustrates how a surface of the desired shape is achieved usinga membrane assembly of the invention. In FIG. 10, the desired shape isspherical, but as described in more detail below the assembly of theinvention can be used to form other shapes; for instance shapes definedby one or a combination of Zernike polynomials. For non-opticalapplications, different shapes may be required. The lens assembly 1 inan actuated state is shown in FIGS. 11 and 12.

FIG. 10 thus shows the membrane 8 of the non-round first lens assembly 1in an actuated state projected onto an imaginary sphere of radius R toafford a positive focal power. The actuation point {circle around (A)}and hinge points {circle around (H)}1, {circle around (H)}2 are shown. Aforce F may be applied at the actuation point {circle around (A)} bymeans of an actuation device connected via the holes 28 a, 28 b.

The lower half of FIG. 10 shows a section on the line b-b of the upperhalf through the optical centre OC at the vertex of the membrane 8 inthe actuated state. The direction of application of the force is shown(downwards in FIG. 10). The membrane 8 is distended in a substantiallypart-spherical configuration, and the edge of the membrane 8 defined bythe supporting rings 2, 10 has a profile that substantially follows thesurface contours of the sphere. In the un-actuated state the membrane 8is flat, and the edge of the membrane (and thus the supporting rings 2,10) is also flat—represented by line L in the lower half of FIG. 10. Inthe actuated state, the membrane 8 substantially follows the surface ofthe sphere, and its edge no longer lies in a plane (as it would do ifthe lens were circular and the membrane formed a spherical cap). Thiscan be seen by comparing the edge of the membrane with the line L. Inthe actuated state the membrane 8 is displaced at the actuation point{circle around (A)} below the line L, representing the plane of themembrane 8 in the un-actuated state, but where the long sides 3, 5 ofthe membrane deviate (inwardly) from a round shape, they are displacedabove the line L, so that a major portion of the edge of the membranewould fit contiguously against the surface of a sphere of radius R.

In FIG. 10 the optical centre OC is located, according to ophthalmicconvention, at a predetermined distance from the centre of the bridge 94of the eyeglasses 94. This distance is half the centration distance,which is the distance between the optical centres of the two lenses 1 ofthe eyeglasses 90, which in turn is the optimum distance for a wearer ofthe eyeglasses. With the shape of lens illustrated, the point OC isapproximately central between the long sides 3, 5 of the lens assembly,but is positioned leftwards of the visually observed geometric centre onthe axis between the short sides (i.e. from eye to nose when worn).

The lens assembly of the present invention is adapted to provide acontinuously adjustable lens power by a desired number of dioptres D,typically 0 to +4D, which is additive with any lensing power afforded bythe front cover plate 4 and/or rear cover plate 16. In general, thepower of a lens D is given by the product of the difference inrefractive index of the lens material and its environment, and thecurvature of the interface. Thus the formula is:D=(n−1)(1/R)  (I)

Where n is the refractive index, 1 is taken as the refractive index ofair and R is the radius of the sphere of which the lens forms part (asillustrated in FIG. 3b ).

In the lower half of FIG. 10, the edge of the membrane 8 is maximallydisplaced at the actuation point {circle around (A)} in the direction ofapplication of the force F. The hinge points {circle around (H)}1,{circle around (H)}2 coincide with points on the edge of the membrane 8(as defined by the supporting rings 2, 10 in the first lens assembly 1)that involve substantially no displacement upon deformation of themembrane 8. It can be seen that these points in the actuated positionhave not moved from and lie approximately on the line L. (Note they areout of plane of the section shown in the lower half of FIG. 10). Inorder to control optimally the deformation of the membrane 8, the hingepoints {circle around (H)}1, {circle around (H)}2 should be locatedwhere minimal movement or no movement of the edge of the membrane 8 isrequired, otherwise the profile of the edge of the membrane woulddeviate at the hinge points {circle around (H)}1, {circle around (H)}2from the desired spherical (or other) shape, resulting in unwanteddistortion of the membrane. Suitably the hinge points {circle around(H)}1, {circle around (H)}2 may be generally equidistant from theoptical centre OC as mentioned above, so that they lie on the samecircular contour of displacement when the lens is actuated, i.e. acontour of no displacement. However, depending on the shape and otherparameters of the lens assembly 1 this may not be possible, and somedifference in the distances between the respective hinge points {circlearound (H)}1, {circle around (H)}2 and the optical centre OC can betolerated, notwithstanding the resulting distortion that will occur inthe vicinity of one or both hinge points {circle around (H)}1, {circlearound (H)}2. In FIG. 10, it can be seen that one hinge point {circlearound (H)}1 is situated further from the centre OC than the other hingepoint {circle around (H)}2, leading to some distortion of the membranein the corners of the lens adjacent the hinge points {circle around(H)}1, {circle around (H)}2, but this is tolerable, provided there is amajor zone around the centre OC where little or no distortion occurs.This is best shown in FIG. 13.

It will be appreciated that maximal displacement of the membrane 8occurs at the actuation point {circle around (A)}, which should alwayslie on the desired locus of displacement of the membrane edge to definea spherical-fitting profile between the un-actuated and maximum focalpower positions. Since the edge of the membrane 8 at the one short side7 of the lens, which includes the actuation point {circle around (A)},happens to be substantially circular should be it follows a circularcontour of displacement when actuated, but again some deviation fromcircular can be tolerated. The actuation point should therefore belocated on the one short side 7 at the point furthest away from theoptical centre OC. Were the particular shape considered here not suchthat a segment of its perimeter formed a circular arc about the opticalcentre, additional actuation point(s) (active or passive) may berequired to maintain the surface fidelity. It will be seen from FIG. 10that in the first lens assembly 1, the points furthest away from thecentre OC are in the corners of the membrane 8, between the long sides3, 5 and the one short side 7—identified as positions {circle around(B)} and {circle around (C)} in FIG. 10. However the actuation point{circle around (A)} is proximate to these points and the stiffening rib3 a serves to distribute the load applied at the actuation point {circlearound (A)} along the one short side 7 of the membrane 8 with anacceptable degree of distortion of the membrane shape.

Those skilled in the art will understand that the optical power of thefirst lens assembly 1 can be varied effectively by varying the radius Rof the sphere, which varies the curvature of the optical surfaceprovided by the flexible membrane 8 and hence adjusts the power of thelens. As R is reduced, the optical power of the lens increases becausethe curvature of the membrane is more pronounced. This is achieved bygreater deformation of the membrane 8, which in turn is effected byincreasing the displacement of the supporting rings 2, 10 at theactuation point {circle around (A)} rearwardly towards the rear coverplate 16, resulting in greater fluid pressure in the cavity and greaterforwards distension of the membrane.

The way that this variable deformation is achieved for the first lensassembly 1 according to the invention is described in greater detailbelow.

FIGS. 3-5 show the first lens assembly 1 is its un-actuated state, andFIGS. 11 and 12 show an exemplary actuated state. In practice the firstlens assembly 1 is continuously adjustable between the un-actuated stateand its maximum deformation; the actuated position of FIGS. 11 and 12 isjust one deformed position which is provided as an exemplar of alldeformed positions. As described above, the width of the supportingrings 2, 10 varies round their extent, while their thickness in thez-direction remains substantially constant. Specifically the rings 2, 10are widest at the short sides 7, 9 of the assembly 1 and becomeprogressively narrower away from those short sides towards the middlesof the long sides 3,5 as best seen in FIG. 9. They are thinnest atpoints {circle around (D)} and {circle around (E)} on the longer sidesintermediate the short sides 7, 9 (see FIG. 10). Note the thinnestpoints are not necessarily symmetrical as between the two long sides;they are thinnest in this region because this is where their bendingneeds to be greatest, as can be understood with reference to FIG. 10described above.

In operation, in order to increase the focal power of the lens assembly1, an actuating force F is applied, directly or indirectly, to thesupporting rings 2, 10 at the point {circle around (A)} on the one shortside 7 of the assembly to move the supporting rings 2, 10, and themembrane 8 clamped between them, rearwardly towards the rear cover plate16. The force is applied about half-way along the one short side 7 andthe actuating device should be arranged to react against the retainingring 6 which is held within the rim 93 of the frame 92 which thus servesas a support.

There are various means by which the actuating force may be applied thatwill be apparent to those skilled in the art; some embodiments aredisclosed below. The force should be applied in a direction that issubstantially normal to the plane of the supporting rings 2, 10. Asdescribed above, the supporting rings 2, 10 are hinged at the two points{circle around (H)}1, {circle around (H)}2 on the other short side 9 ofthe assembly 1. The hinge points are designed to remain stable duringactuation of the lens assembly 1 by means of the formations 39 withinthe retaining ring 6: when assembling the lens assembly 1, the rearcover plate 16, with the dish-shaped part 12 attached thereto, thediaphragm 24 and the supporting rings 2, 10 with the membrane 8 heldbetween them are pre-assembled as a stack and then inserted into theretaining ring 6 and slid under the formations 39 at the hinge points{circle around (H)}1, {circle around (H)}2. The side wall 18 of thedish-shaped part 12 allows a small amount of movement, so that thesupport rings 2, 10 can move closer towards the bottom wall 19 of thedish-shaped part 18 to increase the pressure of the fluid within thecavity, which in turn causes the membrane 8 to distend forwardly towardsthe front cover plate 4, adopting a spherical (or other) shape as shownin FIG. 12, thereby to increase the focal power of the lens, asdescribed above. Even though the membrane is non-round, it is able toadopt the desired spherical (or other shape) form by virtue of theconstruction of the supporting rings 2, 10.

The force applied to the one short side 7 of the supporting rings 2, 10at the actuation point {circle around (A)}, in combination with thehydrostatic pressure applied to the membrane by the fluid within thecavity, causes the supporting rings 2, 10 to bend. FIG. 11 shows thesupporting rings 2, 10 exhibiting a degree of bending upon applicationof the actuating force F. The supporting rings 2, 10 remainsubstantially stationary at the hinge points {circle around (H)}1,{circle around (H)}2 (although there is a degree of local tilting of therings 2, 10 at these points). However, towards the middles of the longsides 3,5 of the assembly including points {circle around (D)} and{circle around (E)}, the rings flex forwards as described above, in anopposite direction to the force F, so that the supporting rings 2, 10adopt a profile that would conform to the surface of a sphere (or otherform) having same shape as the membrane 8. If the supporting rings 2, 10were circular, they would remain planar when the membrane deformsspherically, but the non-round shape of the rings 2, 10 implies thatthey cannot remain flat when the membrane is distended,

The ability of the supporting rings 2, 10 to flex in this manner andthus control the deformation of the membrane 8 to avoid unwanteddistortions of the spherical or other shape is made possible by thepredetermined variation in width of the supporting rings 2, 10 roundtheir extent, and in particular in view of the fact that they are madenarrower at the points where they are required to bend the most to adoptthe desired profile. The predetermined variation in the width of thesupporting rings 2, 10 produces a corresponding variation incross-sectional area of the support rings 2, 10 and thus a correspondingpredetermined variation of the second moment of area of the supportrings. In particular the width of the supporting rings 2, 10 iscontinuously adjusted around the rings and reaches a minimum towards themiddles of the long sides 3, 5 where the bending is thus greatest. Inthe absence of significant variation in other parameters, a differencein the second moment of area results in a difference in the bendingstiffness.

As shown in FIGS. 10-12, the flexible membrane 8 is caused to bulgeforwards in an opposite direction to that of the actuating force F. Whenthe supporting rings 2, 10 are moved closer to the rear of the cavity atthe actuation point {circle around (A)}, the liquid 11, beingessentially incompressible, is forced to occupy a more central region ofthe cavity 22, owing to the elasticity of the membrane 8, thusincreasing the curvature of the optical surface defined by the membrane8 and the optical thickness of the cavity between the membrane 8 and therear supporting plate 16 at the optical centre OC of the assembly, thusproducing a higher power lens. Specifically, the deformation of theflexible membrane 8 is centred on the point OC as shown in FIG. 10 whichthus forms the vertex of the lens.

In prior art fluid-filled lenses, in order to ensure spherical bulgingof the membrane, the membrane is held by a supporting structure that isstiff and circular, so that only a circular portion of the membrane isunconstrained and can bulge forwards upon increasing the pressure offluid. In some lenses (see e.g. GB 2353606 A) this is achieved by makingthe entire lens assembly circular in shape. In other lenses such forexample as the one disclosed in WO 95/27912, the supporting structurecomprises a stiff border around a circular central aperture where themembrane can bulge forwards. In WO 95/27912 the border is wide and bulkyin places, which is aesthetically undesirable. By contrast in thepresent invention, whilst the short sides 7, 9 of the supporting rings2, 10 are somewhat wider than the long sides 3, 5, as can be seen fromFIG. 9, they are still relatively narrow in comparison with the area ofthe lens. Thus from an aesthetic point of view, spherical (or other)deformation of the membrane 8 is achieved without any significantadverse impact on the appearance of the lens assembly 1, which has anon-circular shape and relatively thin edges.

Upon actuation, when the flexible membrane 8 bulges forwards as shown inFIGS. 10 and 11, the amount of fluid 11 held in the cavity 22 remainsconstant, but because the membrane 8 changes in shape from a relativelyflat profile to the distended profile shown, some of the transparent oilis displaced into the central area of the lens. The displacement of theoil causes the membrane to adopt the actuated shape, thus increasing thepower of the lens. The fluid 11 is sealed within the cavity 22 by themembrane 8, the diaphragm 24 and the dish-shaped part 12.

It will be understood by those skilled in the art that the sphericaldeformation of the supporting rings 2, 10 and of the flexible membrane 8that is depicted in FIG. 10 is provided by way of example only toillustrate the change in shape of the various parts of the assembly 1,and that the deformation provided by the assembly of the invention mayvary from that shown. In particular for any given lens assembly 1, themembrane 8 is continuously deformable between its un-actuated position,in which it is planar, and a fully distended position, as determined bythe actual configuration and properties of the materials used for theassembly 1. In each position between the un-actuated position whichprovides no optical power and the fully distended position, the hingepoints {circle around (H)}1, {circle around (H)}2 on the supportingrings 2, 10 remain essentially stationary and at least a major portionor portions of the supporting rings 2,10, including the hinge points{circle around (H)}1, {circle around (H)}2, adopt a spherical (or otherform) profile.

The actual variation in width of the support rings 2, 10 that isrequired to obtain the predetermined variation in bending moment roundthe rings, as described above, may be calculated by Finite ElementAnalysis (FEA). For quasi-static or low frequency optical and otherapplications, static FEA should be employed adequately. However, wherethe surface is intended for acoustic applications, dynamic FEA isappropriate. As those skilled in the art will be aware, FEA—whetherstatic or dynamic—involves numerous iterations performed using acomputer with the input of selected parameters to calculate the membraneshape that would result in practice with an increasing force F appliedat the actuation point(s). The element shape is selected to suit thecalculation being performed. For the design of the rings 2, 10 of thepresent invention, a tetrahedral element shape has been found to besuitable. The selected parameters to be input include the geometry ofthe supporting rings 2, 10, the geometry of the membrane 8, the modulusof the membrane 8, the modulus of the rings 2, 10, including how themodulus of the rings varies round the rings (which may be definedempirically or by means of a suitable formula), the amount ofpre-tension in any of the parts, the temperature and other environmentalfactors. The FEA programme defines how the pressure applied to themembrane 8 increases as load is applied to the rings at the actuationpoint {circle around (A)}.

An example FEA analysis output for a supporting ring is shown in FIG.13. The greyscale shows the degree of displacement of the membrane 8away from its planar un-actuated configuration; contours of displacementare superimposed on the greyscale. The membrane shows maximal forwardsdeformation in its central region and maximal rearwards deformation (inthe direction of the applied force F) at the actuation point {circlearound (A)}, with circular contours proving essentially sphericaldeformation. This figure shows the deformation in 2-dimensions; it willbe understood however that this corresponds to 3D spherical deformationin practice. The first lens assembly 1 of the invention achieves asubstantially undistorted spherical lens, centred on the point OC. Itcan be seen from FIG. 13 that the point OC is different from theobserved geometric centre of the lens 1, which is shown by the pointwhere the vertical and horizontal lines cross. This FEA output isreferred to as the “first FEA output” below.

In order to design precisely the rings 2, 10 for optical use, the outputof the FEA analysis may be approximated to the desired shape of themembrane as defined by a polynomial function. In general terms, theshape of an optical surface may described by one or more Zernikepolynomial functions. These have the general formula Z_(n) ^(±m).Various shapes, as defined by Zernike functions or combinations of morethan one such function, are possible using the present invention. Anexplanation of the various Zernike polynomials can be found inPrinciples of Optics¹ ¹ “Principles of Optics” M. Born and E. Wolf,7^(th) Ed, C.U.P., (1999). ISBN 0-521-64222-1

A priority for ophthalmic applications, for instance, is to be able toachieve vision correction with a linear superposition of Z₂ ^(±2)(astigmatism) and Z₂ ⁰ (sphere for distance correction). Opticianstypically prescribe lenses based on these formulae. Higher ordersurfaces with additional components Z_(j) ^(±j) are also possible inaccordance with the present invention if additional control points (asdescribed below) are provided on the edge of the membrane, where jscales in similar magnitude to the number of control points. Higherorder surfaces with components Z_(j) ^(±k) (k≦j) may also be possiblewhere the shape of the membrane edge permits.

Variants of the first lens assembly 1 of the invention are able toproduce static membrane shapes corresponding to any such polynomial forwhich j=k. Various complex surfaces are known to be possible and usefulfor certain applications. For example, laser vision correction surgeryoften works to certain higher order functions, and thus alternativeembodiments of the lens assembly of the invention might be used as analternative to laser surgery. Various linear superpositions of scaledZernike polynomials of the form Z_(n) ^(±m) are possible:Z ₂ ^(±2) , Z ₂ ⁰ , Z _(j) ^(±j) , Z _(j) ^(±k) (k≦j)

In general, except at their periphery, surfaces achievable by deforminga membrane with pressure may have one or more local maxima or one ormore local minima, but not both, in addition to saddle points. Theshapes that are achievable are necessarily limited by the shape of theperiphery, which is stable in use.

In some embodiments of the lens assembly of the present invention, aspherical Zernike function may be used, but higher spherical orderfunctions can also be used if desired, by creating a shape that is thesum of a number of Zernike polynomials.

The first FEA output is then correlated with the desired Zernikefunction across the membrane (“second polynomial output”) to see howwell the first FEA output approximates to the desired shaped as definedby the chosen Zernike function. Depending how well the first FEA andsecond polynomial outputs correlate with one another, the relevantparameters of the lens can be adjusted to achieve a better fit on thenext iteration. In other words, by seeing how well the simulateddeformation of the membrane 8, as calculated by FEA, approximates to thedesired surface shape as described by the selected Zernike polynomialfunction, one can see how well the chosen supporting ring 2, 10parameters perform. It is possible to determine which regions of thesupporting rings 2, 10 need to be tuned (or which other parametersshould be adjusted) to improve the correlation of the first and secondoutputs.

The above-described iterative process is carried out over a number ofdifferent lens powers so that a lens whose power varies continuouslywith deformation of the supporting rings 2, 10 (and the force F applied)can be designed. This iterative process has been carried out to achievea number of working embodiments of the supporting rings 2, 10 inaccordance with the invention. Thus the supporting rings 2, 10 aredesigned to bend variably round their extent and with respect to theadjustment in lens power required. The variation in width of thesupporting rings 2, 10 in the x-y plane, perpendicular to the opticalz-axis of the lens, round their extent can also be adjusted fordifferent lens shapes, taking into account the locations of the hingepoints {circle around (H)}1, {circle around (H)}2 and actuation point{circle around (A)} relative to the desired optical centre OC.

Once the shape of the membrane 8 has been calculated by FEA as describedabove, the optical properties of the membrane as an optical lens surfacemay be determined by suitable optical ray tracing software (e.g. Zemax™optical software available from Radiant Zemax, LLC of Redmond, Wash.)using the calculated membrane shape. By way of example, FIG. 14 showshow the spherical lens power varies across the membrane 8 of the firstlens assembly 1 when distended, the distended shape being calculated bystatic FEA. The darkest areas show the greatest lens power, and as canbe seen from FIG. 14, the inflated membrane 8 produces a lens surfacewhich has a satisfactorily uniform spherical lens power.

In view of the fact that the degree of deformation of the flexiblemembrane 8 can be adjusted smoothly through a range, the lens assemblyof the invention represents a significant improvement over conventionalbifocal lenses, where the wearer needs to glance downwards to lookthrough the near-vision lens. By using the lens assembly 1 of thepresent invention, the lens power can be adjusted on demand for nearvision and occurs in an optimal region of the lens, namely in the regionof the optical centre. The lens assembly is thus usable for viewing anear object without the need to adjust head position or the direction ofgaze.

FIGS. 15 and 16 show sample FEA outputs from designing the membranereinforcing diaphragm 24. FIG. 15 shows the pre-tension across theflexible membrane calculated by FEA in a lens assembly in accordancewith the invention that is similar to the first lens assembly 1described above, but which omits the diaphragm 24, with the membraneun-actuated. The greyscale reveals significant variation in thepre-tension in the membrane, with several regions of relatively greatertension and several regions of relatively lower tension; the tension inthe membrane is noticeably uneven.

FIG. 16 shows the corresponding FEA output for the first lens assembly 1which includes the diaphragm 24. In this assembly 1 the membrane 8exhibits significantly less variation in pre-tension when un-actuatedthan the one of FIG. 15. Over its area, the membrane of FIG. 15 displaysa 30% variation in pre-tension while the membrane of FIG. 16 has only an8% variation.

FIGS. 17 and 18 show the calculated spherical lens powers for the firstlens assembly 1 and for the similar lens assembly in which the diaphragm24 is omitted. Again, it can be seen that the variation in opticalspherical power is much less in FIG. 18; the greyscale shows muchgreater uniformity.

The reinforcing diaphragm 24 thus provides significant benefits inimproving the uniformity of the pre-tension in the membrane whenun-actuated and the optical spherical power of the membrane whendistended, i.e. actuated, that are independent of the shape of themembrane. Effectively the diaphragm 24 increases the stiffness of thesupporting rings 2, 10 in the x-y plane defined by them withoutsignificantly affecting the stiffness of the rings transverse to theplane in the z-axis. As noted above, the reinforcing diaphragm 24 of theinvention may be advantageously used for this purpose in any fluidfilled assembly with a pre-tensioned flexible membrane of a controllableshape forming a wall of the cavity, such as an optical surface of afluid-filled lens, regardless of the outline shape of the membrane. Thediaphragm 24 may therefore also be used in round fluid-filled lens, forexample.

FIGS. 19 and 20 show schematically the mode of actuation of the firstlens assembly 1. The lens assembly 1 is actuated by “angledcompression”. The front and rear plates 4, 16, the retaining ring 6, thediaphragm 24 and other detailed features are omitted for clarity.

FIGS. 19A and 20A show the lens assembly 1 in its un-actuated state. Inthis condition, the membrane 8 is flat.

In FIGS. 19B and 20B, the lens assembly 1 is actuated to increase itsoptical power by the application of a force F applied to the one side 7of the supporting rings 2, 10 at the actuation point {circle around (A)}in a direction to urge the supporting rings 2, 10 towards the rear wall19 of the dish-shaped part 12. The rear wall 19 of the dish-shaped partis held stationary and thus supported by the rear cover plate 16 andretaining ring 6 (not shown in FIG. 19B). This causes the one side 7 ofthe supporting rings 2, 10 to move closer to the rear wall 19 of thedish-shaped part 12. The other short side 9 of the supporting rings 2,10 is anchored at the hinge points {circle around (H)}1, {circle around(H)}2 by the formations 39. The supporting rings 2, 10 thus tiltrearwardly under the influence of the force F to subtend an acute anglewith the rear wall 19. This tilting movement, which is exaggerated inFIG. 19B, is accommodated by the flexible seal formed by the side wall18 of the dish-shaped part 12. As a result of this squeezing together ofthe supporting rings 2, 10 and the rear wall 19 of the part 12, thehydrostatic pressure within the cavity increases, causing the membrane 8to become distended, flexing convexly outwardly as shown.

In FIGS. 19C and 20C, the actuation force is removed which allows thesupporting rings 2, 10 to return to their un-actuated, relaxed state asa result of their intrinsic resilience. The side wall 18 of thedish-shaped part 12 is thus caused or allowed to uncompress, relievingthe hydrostatic pressure within the cavity. In turn, the membrane 8 isallowed to return to its un-distended un-actuated position.

The lens assembly 1 hereinbefore described operates by tilting the rings2, 10 towards the rear wall 19 of the dish-shaped member 12 to reducethe volume of the cavity 22 and thereby to increase the pressure of thefluid 11, causing the membrane 8 to distend outwardly. However thoseskilled in the art will appreciate that the same principles may beapplied to a membrane assembly in which the membrane supporting ring(s)are tilted or otherwise moved away from the rear wall to increase thevolume of the cavity and thereby reduce the pressure of the fluid,resulting in the membrane caving inwardly. The shape of such a concavemembrane may be controlled in an analogous manner by providing a ring orrings having a variable second moment of area such that upon deformationof the membrane the ring or rings adopt the profile needed to producethe desired predefined form in the membrane.

FIGS. 21 and 22 show a second lens assembly 101 according to theinvention. Each of FIGS. 21A-C shows a cross-sectional view of thesecond lens assembly 101 at a different state of actuation, and FIGS.22A-C show corresponding front elevations.

The construction of the second lens assembly 101 is similar to that oflens assembly 1; parts of the second lens assembly 101 that are the sameas or similar to those of the first lens assembly 1 are not describedagain below, but are referred to by reference numerals that are the sameas the reference numerals for the corresponding parts of the first lensassembly 1 but increased by 100.

The second lens assembly 101 has a square shape. While the first lensassembly 1 uses “angled compression” of the fluid cavity 22 foractuation, the second lens assembly 101 uses “cushion” (or uniform)compression as described below.

FIGS. 21A and 22A show the un-actuated state of the second lens assembly101 in accordance with the invention.

In FIGS. 21B and 22B, the second lens assembly 101 is shown in anactuated state to increase its optical power. However, instead oftilting the supporting rings relative to the rear wall of thedish-shaped part 112 by applying a force to one side of the assembly totilt the rings about hinge points on an opposite side, the supportingrings 102, 110 of the second lens assembly 101 are pushed at a pluralityof actuation points {circle around (A)} that are spaced round the rings,so that at each actuation point the rings are displaced relative to thesupport afforded by the frame 92 towards the rear wall 119 by apredetermined distance according to the desired membrane shape. That is,at each actuation point, the rings 102, 110 are displaced according tothe desired locus of displacement of the rings at those points toachieve the desired membrane shape. The precise location of theactuation points and the amount of their displacement will depend on theoutline shape of the membrane 108, but in general according to theinvention an actuation point should be situated at each point on therings where the displacement is a local maximum. Thus in the second lensassembly 101, an actuation point {circle around (A)} is situated at eachcorner 121 of the membrane 108, and each actuation point {circle around(A)} is displaced by the same amount as the assembly 101 is actuated asthe other points.

Intermediate the corners 121 of the membrane 108, the square outlineshape of the membrane means that it deviates inwardly from a roundconfiguration. This means that when the membrane is distendedspherically, the sides 103, 105, 107, 109 of the membrane should bedisplaced in the z-direction by a smaller amount than the corners 121,so that the sides arch forwardly between the corners 121, and may evenbe displaced forwards relative to the un-actuated position towards thecentre of each side at points {circle around (C)}, {circle around (D)},{circle around (E)} and {circle around (F)} to produce the requiredspherical profile.

In an alternative embodiment, the rings 102, 110 could be heldstationary at the corners 121, e.g. by formations of the kind used inthe first lens assembly 1 for the hinge points {circle around (H)}1,{circle around (H)}2, and an actuating force F applied uniformly to therear cover plate 116 in the z-direction, as shown in FIG. 21B. Areaction force would then be applied to the rings at the substitutehinge points {circle around (H)} in the corners 121 where the rings areheld.

Upon actuating the second lens assembly 101 as described above, theflexible side wall 118 of the dish-shaped part 112 is compresseduniformly, increasing the pressure of the fluid 111 within the cavity122. This causes the membrane 108 to inflate and bulge outwardly in aconvex manner. In spite of the square shape of the membrane, the widthand thus bending modulus of the rings 102, 110 is varied round themembrane such that they deform in a controlled, predetermined manner, ascalculated by FEA for instance, to maintain a spherical (or otherpreselected) profile, such that the membrane is caused to deformspherically (or according to the other preselected profile).Specifically, in the embodiment shown in FIGS. 21 and 22, the rings 102,110 are thicker at the corners 121 than they are between the corners,allowing the rings intermediate the corners to flex forwardly relativeto the corners in the manner described above.

In view of the even movement of the supporting rings 102, 110 towardsthe rear cover plate 116, a smaller total displacement of the supportingrings 102, 110 may be required to inflate the membrane 108 fully ascompared with a similarly dimensioned “angled compression” assembly.Thus the thickness of the second lens assembly 101 may be minimised.

In order to return the second lens assembly 101 to the un-actuatedstate, the actuating force is removed from the actuation points {circlearound (A)} (or from the rear cover plate as applicable) and the ringsare allowed to return to the un-actuated starting position as shown inFIGS. 21C and 22C. In some embodiments, the resilience of thedish-shaped part 112 may be sufficient to restore the rings to theun-actuated state when the actuating force is removed. However, in avariant, the assembly may be actively returned to the un-actuatedposition by driving the rings 102,110 at the actuating points in theopposite direction or by holding the rings 102, 110 and apply a reverseforce—F (see FIG. 21C) to the rear cover plate 116 to pull the plateaway from the rings. The pressure of the fluid 111 within the cavity 122is thus relieved, allowing the membrane and the rings to return to theirplanar configuration.

The first and second lens assemblies 1, 101 are similar to one anotherin that they both require application of a force to compress theassembly. The difference between them resides primarily in the number ofactuation points {circle around (A)} and hinge points {circle around(H)}. In the first lens assembly 1 there is one actuation point {circlearound (A)} on one short side 7 of the assembly and two hinge points{circle around (H)}1, {circle around (H)}2 on the other short side 9that define a tilting axis. The long sides 3, 5 are unconstrained andare free to bow forwards as the cavity 22 is compressed. In the secondlens assembly 101, there are no hinge points, but actuation points{circle around (A)} are provided at each corner 121 where maximalcompression of the cavity 122 is required to achieve the desiredmembrane shape.

In general, the membrane assembly of the present invention utilisessemi-active control of the shape of the supporting rings 2, 10; 102, 110by actively controlling the position of the rings at a plurality ofcontrol points at spaced locations round the rings, which control pointsmay be hinge points or actuation points, and allowing the rings 2, 10;102, 110 to flex freely between the control points. An actuation pointis a point at which the displacement of the rings is either activelycontrolled to achieve compression of the cavity 22; 122, or thedisplacement of the rings is modified by a passive element, a spring forexample. A hinge point is a point where rings are held in a fixedposition, but the rings are allowed to tilt if required to allow thecavity to be compressed by ‘angled compression’ such, for example, as inthe first lens assembly 1. Those skilled in the art will appreciate thatthe region of the rings 2, 10; 102, 110 that is affected by a controlpoint should be as small (localised) as possible, and adjacent controlpoints should not, in general, be rigidly connected to each other, toallow the rings to flex along the rings as required to achieve thedesired shape. Generally there must be at least three control points(hinge points or actuation points) in order to define stably the datumplane of the membrane 8.

There should be at least one control point within each sector of therings 2, 10; 102, 110. By a “sector” is meant a region of the ringsbetween two adjacent unsupported minimal points on the rings 2, 10; 102,110 where the rings approach locally closest to the defined centre ofthe membrane 8; 108. At these minimal points, the displacement of therings 2, 10; 102, 110 towards the rear wall 19 when actuated is a localminimum. In fact, in the embodiments described, the rings 2, 10; 102,110 are actually displaced forwards, away from the rear wall 19 whenactuated, and so in these embodiments the minimal points are actuallypoints of local maximum displacement forwardly relative to the assembly.

The “centre” is the predefined centre of the desired distended shape ofthe membrane. In the case of a lens assembly, the centre may be theoptical centre OC at the vertex of the inflated membrane. Within eachsector, the control point should be positioned at or close to themaximal point at which the rings 2, 10; 102, 110 are disposed locallyfurthest away from the centre; in other words where displacement of therings 2, 10; 102 rearwards towards the rear wall 19 is a local maximumin the actuated state. The rings 2, 10; 102, 110 should be unconstrainedat points intermediate the control points, where the desireddisplacement of the rings 2, 10; 102, 110 towards the rear wall 19 isless than at the neighbouring control points, so that the edge of themembrane 8; 108 may arch forwardly relative to the positions it wouldhave adopted if the rings were inflexible, except short lengths of therings 2, 10; 102, 110 may be supported, e.g. by stiffening ribs such asstiffening ribs 3 a, 3 b, if the supported region of the rings 2, 10;102, 110 does not significant deviate from a circular locus relative tothe optical centre OC. However, the support for the rings should stillallow some flexing of the rings, including in the direction along therings to avoid unwanted distortions.

FIG. 23 shows how the distance between the optical centre OC and therings 2, 10 varies in the first lens assembly 1 round the rings 2, 10.The units in FIG. 23 are arbitrary. It will be appreciated that if themembrane were round, then the plot-line would be flat. As shown in FIG.10, the membrane 8 of the first lens assembly 1 defines two mainsectors—S1, S2. Sectors S1 and S2 are each defined between two adjacentunsupported minimal points {circle around (D)} and {circle around (E)}which, as described above, are disposed approximately midway along thetwo long sides 3, 5 of the membrane 8. Sector S1 includes said othershort side 9 and the maximal point {circle around (H)}1, while sector S2includes the one short side 7 and the maximal points {circle around (B)}and {circle around (C)}. The actuation point {circle around (A)} isdisposed intermediate the two maximal points {circle around (B)} and{circle around (C)}. In a perfect membrane assembly according to theinvention, an actuation point would be provided at each of the maximalpoints {circle around (B)} and {circle around (C)} with point {circlearound (A)} technically being a local minimal point, but for convenienceand practicality, a single actuation is provided at point {circle around(A)} between points {circle around (B)} and {circle around (C)}. As bestseen in FIG. 23, the distance from the rings 2, 10 to the optical centreOC of the membrane is generally constant between the two maximal points{circle around (B)} and {circle around (C)}, and while actuation point{circle around (A)} is technically a minimal point (a local minimumturning point), the displacement of the rings at point {circle around(A)} is still positive ({circle around (A)} is further from the opticalcentre than the hinge points {circle around (H)}1, {circle around (H)}2)and, as a minimal point, it is insignificant in comparison with themajor turning points {circle around (E)} and {circle around (F)}, andthe stiffening rib 3 a serves to support the rings 2, 10 between theadjacent maximal points {circle around (B)} and {circle around (C)}across the minimal point at {circle around (A)} and to distribute theload applied at the actuation point {circle around (A)} along the oneshort side 7 of the assembly.

Sector S1 also includes the hinge point {circle around (H)}2, which isnot disposed at a maximum or minimal point, but helps to define theplane of the membrane for which at least three control points areneeded. In the case of a membrane assembly that operates in the “angledcompression” mode described above, e.g., the first lens assembly 1 ofthe invention, a hinge point can be used at any control point on themembrane supporting rings 2, 10 where the rings do not move (or do notmove substantially) during actuation of the lens. The hinge points{circle around (H)}1, {circle around (H)}2 of the first lens assembly 1are thus disposed within the same sector and define a tilting axis T(see FIG. 10) that is bisected substantially perpendicularly by an axisbetween the tilting axis T and the actuation point {circle around (A)}.The tilting axis T is also generally parallel to the short sides 7, 9 ofthe assembly. The optical centre OC is disposed between the tilting axisT and the actuation point {circle around (A)}. In some embodimentsadjacent hinge points may be situated in adjacent sectors if there is aminimal point between them.

FIG. 24 shows how the distance between the optical centre OC and therings 102, 110 varies in the second lens assembly 101 round the rings102, 110. As can be seen there are four unsupported minimal points{circle around (C)}, {circle around (D)}, {circle around (E)} and{circle around (F)}, where the rings 102, 110 are disposed locallyclosest to the centre OC. The corners 121 of the assembly are furthestaway from the centre OC, and so these comprise maximal points. Anactuation point {circle around (A)} is placed at each corner 121, andthe sides 103, 105, 107, 109 are left unconstrained. The four minimalpoints {circle around (C)}, {circle around (D)}, {circle around (E)} and{circle around (F)} define four sectors S1-S4, and a respective one ofthe actuation points {circle around (A)} is disposed within each sector.In the alternative embodiment where an actuating force F applieduniformly to the rear cover plate 116 in the z-direction, as shown inFIG. 21B, a hinge point {circle around (H)} may be placed in each corner121, and this is possible because the effective displacement of therings 102, 110 in each corner 121 is the same, so the effectivedisplacement at each hinge point {circle around (H)} is the same.

It will be understood that the more control points that are provided,the more accurately the deformation of the membrane can be controlled.Furthermore, additional actuation points facilitate improved control ofthe membrane surface and a wider set of possible lens shapes.

It will be understood by those skilled in the art, that if lensassemblies 1; 101 of the type described herein are used in a pair ofeyeglasses, such as eyeglasses 90 of FIGS. 1 and 2, a selectivelyoperable actuation mechanism should be provided to afford the necessarycompression of the cavity 22, 122 and fluid pressure adjustment tooperate the lens, either directly or indirectly. Such an actuationmechanism may be conveniently provided either in the bridge 94 or one orboth of the temple arms 93. In some embodiments a separate actuationmechanism for each lens assembly 1; 101 may be provided in each arm 93,and the mechanisms linked electronically to provide simultaneousactuation of the two assemblies 1; 101. The actuation mechanism is notdescribed herein, but in general terms may be mechanical, electronic,magnetic, automatic with eye or head movement, or involve use of a phasechange material, such as shape memory alloy (SMA), wax, or anelectroactive polymer. In the event that some passive control of thelens assembly 1; 101 is desired, the fluid pressure could be adjustedwith a pump.

It will be appreciated that the use of separate front and rearsupporting rings 2, 10; 102, 110 is not essential to achieve the basicfunctionality of the lens assembly 1; 101 of the present invention, andin some variants the membrane 8; 108 may be supported by a singleflexible ring. However, it has been found that the use of two or moresupporting rings is advantageous for controlling for example the rate oftwist in the supporting rings 2, 10, and particularly during manufactureof the assembly.

FIG. 25 illustrates the attachment of a flexible membrane 208 to asingle membrane supporting ring 210 using an annular layer 254 ofadhesive. It has been found that when a membrane 208 is attached to asingle ring 210 with adhesive in this manner, the tension that isimparted to the membrane 208 causes the membrane 208 to exert a momentaround the support ring 210 and pull on one face of the support ring 210thereby tending to tilt the supporting ring 210 locally towards thecentre of the lens, as shown in dotted lines in exaggerated form. Thisis undesirable because it means that the ring 210 does not sit squarelywith the other components of the assembly and makes it more difficult tocontrol bending of the ring 210. Such unwanted torsion in the ring 210also gives rise to edge effects in the lens and the introduction ofoptical aberrations as a function of the lens power.

The present invention provides a solution to this problem by using twosupporting rings 2, 10; 102, 110; 302, 310 (see FIG. 26). FIG. 26 showsan improved assembly method in which a flexible membrane 308 is heldbetween the front and rear supporting rings 302, 310. In this improvedmethod, the membrane 308 is pre-tensioned as before, but as well asapplying a layer of adhesive 354 to a front face of a rear support ring310, a layer of adhesive 356 is also applied to a rear face of a frontsupporting ring 302. This can be done simultaneously or sequentially.The two supporting rings 302, 310 are then brought togethersimultaneously on either face of the membrane 308 as shown to sandwichthe membrane 308 therebetween. Since the flexible membrane 308 is neverheld on just one of the rings, the additional support provided by bothrings 302, 310 at once balances any local torsional forces that wouldotherwise occur, therefore providing balanced support. The adhesive isthen cured. Thus a substantially sandwich planar structure which holdsthe pre-tension in the membrane 308 is formed. Those skilled in the artwill appreciate that more than two supporting rings can be employed ifdesired, provided that the membrane is sandwiched between supportingrings in such a way that the tension in the membrane is applied evenlyto the rings on each side of the membrane to avoid unwanted torsionalforces. Thus, for instance, two or more supporting rings may be providedon each side of the membrane.

Various embodiments and aspects of the present invention are describedabove, all of which provide for controlled deformation of the flexiblemembrane 8, 108. In particular, described embodiments show howsubstantially spherical deformation, or deformation according to one ormore Zernike polynomials or similar surface expansions, of the elasticmembrane 8, 108 can be achieved. Optical distortion is minimised and thelens can be used to provide a smooth transition from long-distance toshort-distance focus. Such controlled deformation has not been achievedby any prior non-round fluid filled lenses. It will be understood bythose skilled in the art that deformation according to a Zernikepolynomial is not essential, and the present invention can be used tocontrol deformation of an elastic membrane 8, 108 to other desiredshapes. The lens assembly of the invention can be used to correctvarious optical aberrations which may arise depending on theapplication. This can be achieved by design based on combinations ofdifferent Zernike functions.

In the first and second lens assemblies 1; 101 described above, thevariation in stiffness of the membrane supporting rings 2, 10; 102, 110round their extents is achieved by varying the width and hence thesecond moment of area of the supporting rings round the rings, while thedepth of the rings in the z-direction remains substantially constant.This stiffness could be adjusted in different ways: for instance,instead of varying the width of the rings in the x-y plane, the depth ofthe rings in the z-direction could be adjusted. In another alternative,the ring or rings could comprise an assembly of multiple ring segments,each part being formed from a material of selected stiffness and theparts being joined end to end to form the ring. The use of differentmaterials for different segments of the ring would thus allow thestiffness of the ring to be adjusted as desired round the ring. The ringsegments could have the same or different lengths as needed; forinstance shorter ring segments would be used in regions of the ringwhere the stiffness was required to vary more with distance. In yetanother alternative, heat or chemical treatment of selected regions ofthe ring or rings could be used to alter their material properties. Yetanother alternative would be to use a composite material for the ring orrings and to vary the properties of the material at selected locationsround the ring(s) by altering the structure of the material, e.g. bychanging the orientation of reinforcing fibres.

The first and second lens assemblies 1; 101 may suitably be installed ina pair of eyeglasses 90 such that the flexible membrane 8, 108 bulgesforwards away from the wearer's eyes when actuated. This may bepreferred for safety reasons, but it will be appreciated that the lensassemblies 1; 101 could equally well be installed in eyeglasses so thatthe membrane bulges towards the user's eyes.

In the first and second lens assemblies 1; 101 the cavity 22; 122 isdefined in part by the dish-shaped part 12; 112, the rear wall 19; 119of which is attached to the rear cover plate 16; 116. In a variant, thedish-shaped part 12; 112 may be omitted and replaced by a flexiblesealing ring (not shown) which is similar to the side wall 18; 118 aloneof the dish-shaped part and forms a seal between the rear cover plate16; 116 and the rear supporting ring 10; 110 (or the reinforcingdiaphragm 24 if included).

It should also be noted that a fixed prescription lens (for distance ornear vision) could be included in the lens assembly 1; 101 of theinvention. This could be achieved by using a fixed power lens as thefront cover plate 4; 104 and/or as the rear cover plate 16; 116. Such afixed power lens should have an optical centre that is closely alignedwith the optical centre of the adjustable lens OC when actuated.

The adjustable lens assembly 1; 101 of the present invention ashereinbefore described is capable of providing a variation in opticalpower from −8 to +4 dioptres. If a negative lens power is required, theflexible membrane 8; 108 should be arranged to flex inwardly to achievethis.

The present invention may also be used for controlling the deformationof a surface in other fields such, for example, as acoustics. By rapidlyoscillating the applied force, F, oscillating pressure waves will begenerated in a fluid placed in contact with the membrane. Since thedeformation of the membrane can be controlled to be spherical inaccordance with the invention, such pressure waves will appear to haveoriginated from a point source. This ensures that the waves do notexhibit undesirable interference patterns, whilst allowing a loudspeaker(for instance) incorporating the membrane as the transducer to benon-round in form, thus allowing it to be packaged within a confinedspace, for example in a television or mobile phone. In general terms,the above described principles can be applied to any application inwhich the geometry of a surface needs to be controllably varied.

The invention claimed is:
 1. A deformable membrane assembly comprising:an at least partially flexible fluid-filled envelope, one wall of whichis formed by an elastic membrane that is held around its edge by aresiliently bendable supporting ring, a fixed support for the envelopeand selectively operable means for causing relative movement between thesupporting ring and the support for adjusting the pressure of the fluidin the envelope, thereby to cause the membrane to deform; wherein thebending stiffness of the ring varies round the ring such that upondeformation of the membrane the ring bends variably to control the shapeof the membrane to a predefined form, and the moving means comprise aplurality of ring-engaging members that are arranged to apply a force tothe ring at spaced control points; wherein there are at least threecontrol points, and there is a control point at or proximate each pointon the ring where the profile of the ring that is needed to produce thepredefined form upon deformation of the membrane exhibits a turningpoint in the direction of the force applied at the control point betweentwo adjacent points where the profile of the ring exhibits an inflectionpoint or a turning point in the opposite direction.
 2. The deformablemembrane assembly as claimed in claim 1, wherein the moving means applya force to the ring at each control point in the same direction.
 3. Thedeformable membrane assembly as claimed in claim 1, wherein the movingmeans are configured for compressing the envelope.
 4. The deformablemembrane assembly as claimed in claim 3, wherein a control point isdisposed at or proximate each point on the ring where the profile of thering when actuated exhibits a local maximum displacement in the inwardsdirection relative to the envelope intermediate two adjacent points onthe ring where the profile of the ring in the direction exhibits a localminimum displacement in the inwards direction.
 5. The deformablemembrane assembly as claimed in claim 1, wherein the moving means areconfigured for expanding the envelope.
 6. The deformable membraneassembly as claimed in claim 1, wherein the predefined membrane shape isspherical or a form defined by one or more Zernike polynomials Z_(j)^(±k) (k≦j).
 7. A deformable membrane assembly comprising: afluid-filled compressible envelope, one wall of which is formed by adistensible membrane that is held around its edge by a resilientlybendable supporting ring, a fixed support for the envelope andselectively operable means for compressing the envelope in a firstdirection against the support to increase the pressure of the fluidtherein to cause the membrane to deform outwardly in a second oppositedirection; wherein the bending stiffness of the ring varies round thering such that upon distension of the membrane the ring bends variablyto control the shape of the membrane to a predefined form, and aplurality of ring-engaging members are arranged to engage the ring atselected spaced control points for applying the compressive forcebetween the ring and the support; wherein there are at least threecontrol points, and there is a control point at or proximate each pointon the ring where the displacement of the ring in the first direction isa local maximum intermediate two adjacent points on the ring where thedisplacement of the ring in the second opposite direction is a localmaximum.
 8. The deformable membrane assembly as claimed in claim 7,wherein one or more of said control points are actuation points, wherethe ring-engaging members are configured actively to displace thesupporting ring relative to the support.
 9. The deformable membraneassembly as claimed in claim 8, wherein the membrane is continuouslyadjustable between an un-actuated state and fully deformed state, and ateach position between the un-actuated and fully deformed states thesupporting ring is displaced at the or each actuation point by thedistance required to achieve the predefined form of the membrane. 10.The deformable membrane assembly as claimed in claim 9, wherein thesupporting ring is formed with a protruding tab at the or at least oneof the actuation points for engaging the ring with the ring engagingelement.
 11. The deformable membrane assembly as claimed in claim 7,wherein one or more of said control points are hinge points, where thering-engaging members are configured to hold the supporting ringstationary relative to the support.
 12. The deformable membrane assemblyas claimed in claim 11, wherein the membrane is continuously adjustablebetween an un-actuated state and fully deformed state, and thesupporting ring is required to remain stationary at the or each hingepoint to achieve the predefined form of the membrane at each positionbetween the un-actuated and fully deformed states.
 13. The deformablemembrane assembly as claimed in claim 11, wherein two adjacent hingepoints define a tilting axis, and there is at least one actuation pointwhere the ring engaging member is configured actively to displace thesupporting ring relative to the support for tilting the ring relative tothe support about said tilting axis for adjusting the volume of theenvelope.
 14. The deformable membrane assembly as claimed in claim 13,wherein the supporting ring is generally rectangular, having two shortsides and two long sides; the at least one actuation point is located onone of the short sides, the two adjacent hinge points are located on orproximate to the other short side.
 15. The deformable membrane assemblyas claimed in claim 14, wherein the predefined form has a centre, theone short side generally follows the arc of a circle that is centred onthe centre, and the at least one actuation point is locatedsubstantially centrally on said one short side.
 16. The deformablemembrane assembly as claimed in claim 11, wherein said predefined formhas a centre and there are a plurality of hinge points that aresubstantially equidistant from the centre of the predefined form. 17.The deformable membrane assembly as claimed in claim 7, wherein thesupporting ring is free to bend passively relative to the supportbetween the control points.
 18. The deformable membrane assembly asclaimed in claim 7, wherein stiffening elements are provided forstiffening one or more regions of the supporting ring.
 19. Thedeformable membrane assembly as claimed in claim 7, wherein thesupporting ring comprises two or more ring elements, and the membrane issandwiched between two adjacent ring elements.